This is one of those things where the only 100% accurate answer is "it depends".
Most casinos (at least, the ones here) won't let anyone other than staff and players near the table, so it's kind of a moot point. Also, most of the time you won't know which table you're being seated at so how do you know you're watching the right table?
Funnily, it's actually more likely to be allowed at higher stakes tables, where they tend to have a more anything-goes approach as long as your money is good and the players at the table don't object.
In general though, I wouldn't be doing it and I wouldn't be comfortable if someone was randomly standing around behind me while I was playing.